TOPIC: Setting idle on a B20F D-Jetronic FI system

In the forty-plus years I have owned my ’72 1800E, I don’t think I ever knew the proper way to set the idle. Now that I do know how to do it properly and know where to find the idle adjustment screw, I can’t believe how much more smoothly the B20F idles!

You probably thought the idle adjustment screw is on the side of the plenum, right next to the throttle valve, right? If you did, then we were both wrong! That is the throttle valve stop, and it should never be used to adjust the idle. As you can clearly see in the image, this throttle stop screw may have been used to adjust the idle once or twice in the past.

Ah, you noticed the beaded chain and cable. That's the control cable from the cruise control.

Here’s the issue; there are several things that have to work in conjunction for this part of the FI system to function properly. The throttle must be set so that it closes, but doesn’t bind. Once that is set by the throttle stop screw, the throttle valve switch has to be adjusted to sense this closed position. By later adjusting the throttle valve stop screw, you have now opened the throttle valve slightly and disturbed the throttle valve switch so that it no longer will accurately determine the closed valve (which probably doesn’t close completely anymore). I’ll explain why this is important later.

So what is the proper way to set the throttle valve, the throttle valve switch and set the idle? There is actually a procedure to follow and it must be done in a specific order. Let’s do it.

The first thing that must be done and this is the foundation for all other adjustments, is to set the throttle valve properly. To do this, loosen the nut on the throttle stop screw and back it out a turn or so, feeling the throttle valve for when it seats against the stop. Rotate the valve to make sure it is not sticking and the cable is not binding. If either is the case, then that must be corrected before the next step. You may have to remove the inlet duct from the end of the plenum to clean the valve.

Assuming the valve moves freely, adjust the screw until it just touches the valve stop and then turn about ¼ - ½ turn more and lock it down. Again, check that the valve does not stick during its’ range of movement. This is the last time you touch this screw! Next is the throttle valve switch, but you may want to open it up and clean the switches and contacts first.

The valve switch has five rather thin and probably brittle wires in it so be very careful you don’t break any one of them or three. Don’t ask how I know this, but take it as advice from an authority on the subject. If you do happen to break a few wires, you won’t find a wiring diagram anywhere, so I’ll save you the effort and frustration of searching and put one here. An ice cold Guinness would be very much appreciated.

Throttle Valve Switch wiring.

Looking down on the switch, there are three arched lands. The outer land is connected to the left terminal, the center land to the second terminal and the inner land to the third terminal. One wire from the shut-off switch is connected to the third terminal as well and the other to the fourth terminal.

Reconnected Throttle Valve Switch wiring.

To remove the Throttle Valve Switch, release the cable first. There are four spade connectors in the Throttle Switch side. The top two are connected to the shut-off switch which we will use to set the Throttle Valve Switch position after it is remounted. You can also access the terminals at the end of the shut-off switch internally for this operation and you may have to if the shut-off switch must be adjusted. Next , remove the top and bottom screws and pull the switch housing straight out from the valve stem. If it doesn't want to move, you may have to remove the cover and spray a little WD-40 on the shaft. Be careful and don't try to pry it off, else something may break.

When removed, carefully clean the internals with carburetor cleaner or the like. Remove the center spindle and washer, clean and set aside. The upper switch and wipers are the acceleration switch and slip contacts. The lower switch is the fuel shut-off switch. The Zig-Zag (as Volvo called it) will likely show wear from the wiping contacts. These can be cleaned lightly with an eraser (it is fine to clean the Zig-Zag with a soft eraser in this case as they are copper and not gold plated.) and the contacts can be burnished with a strip of paper. Do not use any type of abrasive or sand paper! Be careful not to bend the contacts. Note the screw under the shut-off switch; you’ll need to loosen this mounting screw if the switch has to be adjusted.

Wires soldered and contacts cleaned

Refit the spindle and locate the tab in the wiper frame. Install the Throttle Switch on the valve shaft and re-install the upper and lower screws, leaving them loose enough to rotate the switch body. Connect an ohm meter to the shut-off switch contacts as in the photo below.

Ohm meter leads connected to the fuel shut-off switch contacts.

If you have re-installed the throttle switch with the cover in place, then you will connect the ohm meter to the top two contacts in the connector shown at the left of the throttle switch body. All other adjustments are the same.

The next step will involve rotating the switch body to open and close the shut-off switch contacts. Closed contacts will read 0 ohms and open contacts will read infinity. With the Throttle Valve in the closed position, rotate the switch body clockwise until the meter reads infinity ohms. Very slowly, rotate counter-clockwise until the switch just makes contact and the meter reads 0 ohms. You can mark the plenum casting at the top of the switch body at the graduated scale for a visual reference. Continue rotating the switch body counter-clockwise about 1 degree further, about ½ graduation mark and secure the screws.

Now rotate the Throttle Valve slowly until the meter reads infinity. At this point, there should be about .020” gap between the throttle stop and the stop screw. Relax the throttle valve and check that the shut-off switch is again making contact and that the upper acceleration switch is not making contact. If the upper switch is closed, then the lower switch will have to be adjusted downward slightly. Repeat the above steps rotating the switch body to set the shut-off switch.

In operation, when the vehicle is under power and/or accelerating, the upper acceleration switch is closed and the bottom fuel shut-off switch is open. When the throttle is released and the engine is coasting or braking, the throttle valve and the fuel shut-off switch close, stopping fuel delivery until the engine reaches about 1000 RPM, at which point fuel delivery resumes to bring the engine to a smooth idle.

Now you can probably understand why you must never adjust the Throttle Valve Stop Screw. Adjusting the stop screw will open the throttle valve slightly, which will increase idle speed, but it also changes the setting on the Throttle Valve Switch causing the fuel shut-off switch to open and never allowing it to close. During engine coast or braking, fuel will not shut off and continues to spray. It is of course ignited, but the energy generated is wasted, engine breaking efficiency is diminished and fuel economy is negatively impacted.

The image below shows the condition of my fuel shut-off switch with the throttle valve closed. This switch never made contact and fuel delivery was not stopped, all because I adjusted the valve stop screw thinking it was the idle adjustment screw. How much did this impact my fuel economy?

How many years has this not made contact?

Ok, that's great, but where is this Idle Adjustment Screw?

Well, it happens to be hiding under the inlet duct. And it's not really an adjustment screw, but an adjustable bypass valve. What it actually does is adjust the amount of air allowed to bypass the throttle valve. By allowing more air to bypass, engine speed increases.

The illusive Idle Adjustment Screw

To set the idle adjustment, the engine must be at operating temperature. If you try to set the idle on a cold engine, you might as well pound sand. If the engine is still cold, the auxiliary air regulator is opened and it is allowing air to bypass the throttle valve. So you can adjust all you want and it won’t make a bit of difference until the air regulator closes. That should happen when the engine reaches about 140 degrees F. To check this, pull the front hose off of the plenum that attaches to the Air Regulator when the engine is warm and there should be no vacuum being drawn. If there is, then you must clean or replace the air regulator before you can set idle on a warm engine.

Once again, assuming you are good to go with the air regulator, loosen the lock nut on the idle screw and adjust for an idle speed of 900 RPM for manual transmission or 800 RPM for an automatic. Tighten the lock nut, replace the inlet duct and go drive that puppy!

I want to die like my Grandfather did, peacefully in his sleep.
Not screaming in terror like the passengers in his car.